Conventionally, various measures have been devised for safely continuing running, without damaging the tire further and without disadvantages such as poor steering. This occurs over a distance from the place where the tire was punctured to a place desired by the driver, e.g., a service station where the tire can be changed, at the time when the internal pressure decreases or becomes zero (hereinafter, "punctured") for some reason, and in many cases, due to a foreign object such as a nail or a metal piece piercing the tire, during traveling of the pneumatic tire installed on a vehicle.
Among these measures, as a simple and effective measure for radial tires for passenger vehicles in particular, Japanese Patent Application Publication No. 52-41521 discloses art relating to a so-called side reinforced run-flat tire in which a rubber reinforcing layer having a crescent moon shaped crosssection is made to line the carcass inner peripheral surface of the sidewall, which is the region in the tire having the lowest rigidity, so that the entire sidewall has approximately the same thickness and so as to provide rigidity.
In this side reinforced run-flat tire, in an ordinary state, the load is mainly supported by internal pressure. When the tire is punctured, the supporting of the load is taken over by the rigidity inherent in the reinforced sidewall.
However, in this side reinforced run-flat tire, as illustrated in FIG. 2, when the tire is punctured, buckling of a sidewall S is avoided, but a crown portion 21 exhibits the phenomenon known as buckling, and a tread 22 is thereby lifted up from a road surface R. As a result, a portion 23 of the sidewall separated from the tread 22 contacts the ground.
In this ground-contacting state, obviously, stress concentrates and defects arise at an early stage at portions at which there is much bending deformation. As a result, in order to increase the durability of the run-flat tire, buckling of the crown portion 21 must be suppressed, and tire deformation must, as much as possible, be maintained the same as before the puncture.
Japanese Patent Application Laid-Open No. 6-191243 and Japanese Patent Application Laid-Open No. 6-191244 disclose techniques in which at least one organic fiber cord layer, which is effective for weight reduction, is disposed at the outer periphery of the belt layer, so as to increase the rigidity of the tread base portion.
However, these disclosed techniques do not exhibit sufficient effects for suppressing buckling, and the current situation is that, even if the rigidity of the tread base portion is increased even more, by winding a plurality of layers, the effects are small in proportion to the increase in cost and in weight.